Three-phase electrical distribution networks, for example distributing an electrical power supply through a building, are subject to harmonic currents generated by non-linear loads such as electronic equipment and equipment that uses different kinds of arc processes. Such equipment can generate excessive harmonic currents in the distribution network, including zero phase sequence (triplen) harmonics (3rd, 9th etc.) and conventional harmonics (5th, 7th, 11th, 13th, 17th, 19th etc.).
Zero phase sequence harmonics, together with unbalanced portions of the fundamental and other harmonic currents, are additive in the neutral conductor, which can result in cumulative currents well in excess of the anticipated phase currents and overload the neutral conductor, which is not protected. In addition to the possibility of overload, these harmonics result in high common-mode noise level (neutral to ground voltage), increased total harmonic distortion level, voltage imbalance, increased power losses and other problems which are well known.
Different kinds of L-C filters can be used in low voltage systems, tuned to different harmonic frequencies. These devices present problems which are well known.
In a three-phase distribution network, zero phase sequence harmonics are conventionally controlled using zero phase sequence filters. Such filters, for example so-called "zig-zag" reactors, have a low impedance to zero phase sequence currents and as such serve to attract these currents and effectively divert them from the distribution network. Because the two windings on each core leg have opposite polarities the zero phase sequence current fluxes generated by the windings on any particular core leg are opposite and therefore cancel. However, such filters do not reduce levels of conventional harmonics.
Different kinds of phase shifters are available which allow the creation of quasi-multiphase systems, reducing certain harmonic levels. Cancellation of certain harmonic currents generated by different loads depends upon the degree of phase shifting. However, these devices have a relatively high zero phase sequence impedance, and accordingly do not reduce voltage distortion levels created by zero phase sequence harmonic currents.
Combined zero phase sequence filters have recently become available, which reduce zero phase sequence harmonics and, by virtue of a 30.degree. phase shift between the line and load sides of the filter, cancel or reduce 5th, 7th, 17th, 19th etc. harmonics. However, such devices do not reduce levels of 11th, 13th etc. harmonics. Moreover, this type of filter is directional, in that it has different impedances on different outputs; while this feature may be advantageous in some applications, in general it is preferable to have the lowest possible zero phase sequence impedance on all outputs.
The present invention overcomes these disadvantages by providing a three-phase phase shifting transformer or autotransformer with low zero phase sequence impedance. This is accomplished by interconnecting multiple windings in a secondary winding such that for each output having reduced zero phase sequence impedance the number of ampere-turns of windings creating fluxes in the positive direction is substantially equal to the number of ampere-turns of windings creating fluxes in the negative direction.
For at least one output having reduced zero phase sequence impedance the windings are distributed amongst all three core legs, one winding being oriented in one direction on the core and the other two windings being oriented in the other direction on the core, such that the total number of turns of the other two windings substantially equals the number of turns of the one winding. Thus, the present invention is able to create any necessary phase shifting angles between different harmonic sources, with some or all outputs each having a very low zero phase sequence impedance, so that the invention also acts as a zero phase sequence current filter. This significantly decreases power losses, partly because the cancellation of zero phase sequence fluxes in the core substantially reduces induced zero phase sequence harmonic currents in the primary, which reduces energy usage, avoids overheating and decreases voltage distortion.
These and other advantages will be apparent from the description of the invention which follows.
The present invention thus provides a phase shifting transformer or autotransformer for a three-phase power distribution network, comprising three core legs, and for each phase, a secondary winding comprising a plurality of windings electrically connected in series and distributed amongst the core legs, each secondary winding having at least two outputs for connection to one phase of the power distribution network, such that for at least one output each core leg is provided with at least one winding associated with said at least one output, wherein the number of turns of the windings and their placement and orientation on each core leg is such that, for each said at least one output, the total number of turns generating zero phase sequence flux in the positive direction substantially equals the total number of turns generating zero phase sequence flux in the negative direction.
The present invention further provides a phase shifting transformer or autotransformer for a three-phase power distribution network, comprising three core legs, and a secondary winding having at least two outputs for connection to each phase of the power distribution network, the secondary winding comprising a plurality of windings electrically connected in series and distributed amongst the core legs such that for at least one output each core leg is provided with at least one winding associated with said at least one output, wherein the number of turns of the windings associated with said at least one output and their orientation on the core is such that the total number of turns of windings associated with said at least one output generating zero phase sequence flux in the positive direction substantially equals the total number of turns of windings associated with said at least one output generating zero phase sequence flux in the negative direction.
The present invention further provides a phase shifting transformer or autotransformer for a three-phase power distribution network, comprising three core legs, and for each phase, a secondary winding comprising a plurality of windings electrically connected and distributed amongst the core legs, the secondary winding associated with at least one phase having at least two outputs for connection to one phase of the power distribution network, said at least two outputs having a phase shift no greater than 30 degrees, such that for at least one of said at least two outputs each core leg is provided with at least one winding associated with said at least one output, wherein the number of turns of the windings and their placement and orientation on each core leg is such that, for said at least one output, the total number of turns generating zero phase sequence flux in the positive direction substantially equals the total number of turns generating zero phase sequence flux in the negative direction.